1. Field of the Invention
The present invention relates generally to food product comprising a true oilseed protein curd substantially free of undesirable flavor, color and odor causing compounds. Produced from defatted and/or undefatted oilseeds, the oil content may be regulated so as to produce a variety of textures and consistencies ranging from a firm, chewy meat-like product to a softer, moister, spongy product. Lacking undesirable flavor, odor and color causing compounds, the food product is bland in taste so as to permit its flavor and color enhancement by addition of desired seasonings and food colors. This invention also relates to methods useful in making the desired food product.
2. Related Art
It is the consensus of authorities in the area of world food requirements that oilseed proteins for direct consumption in human foods are the most promising means of solving the protein shortages that exist in the diets of over half the world's population (NSF/MIT Protein Resources Study, 1975).
Soybeans, cottonseed, peanuts, sunflower seeds, sesame seeds and safflower seeds have been known for some time to be valuable and plentiful sources of protein. They are of excellent nutritional value in that they provide high protein with low carbohydrate constituents. Due to their abundance in nature, they also possess the potential of being relatively economical sources of protein. The low fat aspect of food ingredients made from proteins isolated from these oilseeds make them especially popular in today's health conscious society.
Soybeans are by far the world's leading oilseed crop and account for 53% of the total oilseed tonnage grown, supplying 67% of the world's feed protein resources and 35% of the edible vegetable oil production. Unfortunately, phenolic compounds and enzymes present in the soybean cause undesirable bean-like or painty flavor and odor in the final soybean protein products conventionally manufactured.
Objectionable flavor has been cited as one of the main factors limiting the use of soybean products in foods (Beckel and Smith 1944, Kalbrener et al. 1971).
Several types of compounds have been charged as being responsible for soybean's characteristic beany and bitter taste. Phenolic compounds, enzyme systems, aliphatic carbonyls, volatile fatty acids, amines, esters and alcohols are identified most frequently as the problem source. Phenolics also cause adverse color, flavor and anti-nutritional problems in the other oilseeds such as sunflower.
A significant part of the undesirable flavor and odor in soybeans is caused by the enzyme lipoxygenase, which catalyzes the oxidation of polyunsaturated oils and fats which are also present in the whole seeds. The reaction takes place quickly whenever:
(1) the seed structure is damaged, as when the seed is bruised, cracked, or ground so that the enzyme, lipoxygenase and oil are permitted to contact each other; and
(2) a minimum amount of water is present during extraction of oil and protein.
Therefore, care must be taken when the beans are ground and comminuted; otherwise, the enzyme reaction will result in a poor-tasting final product. (U.S. Pat. No. 3,901,978--Nelson et al., 1975).
In recent years, consumption of high-protein, wet-curd type products from soybeans (especially tofu) has increased significantly in the United States despite their characteristic soybean taste. Such foods, though initially confined to the health foods market, now are expected to become dietary staples at some American tables. By 1982, the annual production of tofu in the United States had risen to 27,500 tons, with sixteen percent of the product being sold through restaurants. The manufacture of high-protein wet curds has emerged from a cottage industry to a line of consumer-oriented packaged products sold competitively in supermarkets. For example, the largest domestic tofu manufacturer now produces over 200,000 pounds weekly.
The development of edible protein products from cottonseed, the world's second most important oilseed, has been impeded primarily by two factors: the presence of gossypol and the primary importance placed on the economic value of cottonseed oil. Gossypol, a highly reactive, yellow polyphenolic binaphytaldehyde, occurs in pigment glands which appear as dark specks in ordinary cottonseed. Gossypol is toxic to nonruminant (monogastric) animals including humans, and thus produces adverse physiological effects when ingested. It also imparts a yellow undesirable color to cottonseed protein products.
In the early 1950s, USDA cotton geneticists discovered that by making selections and crosses of cottons grown by the Hopi Indians they could produce cotton which contained few, if any, pigment glands in the seed. Thus, pigment glands and gossypol can be eliminated through breeding to produce "glandless cottonseed".
Hence, the introduction of glandless cotton, which is essentially free of pigments (and gossypol) has enabled glandless cottonseed protein to be beneficially consumed by humans. Researchers have demonstrated the utility and nutritional advantages of glandless cottonseed protein and protein derivatives in a wide variety of food products and food ingredients.
Glandless cottonseed contains two distinct protein components known as storage (SP) and nonstorage protein (NSP). Storage protein is considered as originating from discreet bodies deposited within the seed while the non-storage protein is regarded as the cement which holds the different structures in the seeds together. Nonstorage and storage protein fractions have different functional and nutritional properties which offer considerable latitude in the applications that can be made of them. Nonstorage protein isolates possess better whipping properties than storage protein isolates while storage protein has superior heat gelation properties (Lawhon and Cater, 1971). NSP isolates contain low molecular weight, water-soluble proteins, and have a minimum water solubility at pH 4. SP isolates contain high molecular weight proteins and have a minimum water solubility at a pH of 7 (Martinez et al., 1970).
Storage protein isolates have proven highly satisfactory in bread fortification which can double the protein content of a loaf of wheat bread without affecting its taste or structure. Since they are highly soluble at lower (acidic) pH levels, they are also suitable for the protein fortification of beverages (Lusas et al., 1977).
Most of the prior art processes have attempted to provide an acceptable food substitute or supplement from the aforementioned types of oilseeds which have the desired properties of smooth texture, light color, and bland taste with no odor. For the most part, however, attempts to reach these goals have been unsuccessful. Secondly, the complexities that have been involved in providing these qualities have only been possible through extensive processing steps which increase the cost of the final product.
Oilseed protein isolation processes following the conventional art generally use defatted flakes, meal or flour as a starting material. Protein is extracted from the defatted material using a dilute alkali and then the insoluble residuals are separated by centrifugation. The pH of the liquid extract is then adjusted with an appropriate acid to the point of minimum protein solubility to precipitate the maximum amount of protein. Precipitated protein, also known as curd, is generally adjusted to slightly below neutral pH and spray-dried.
Stephen C. P. Hwa, U.S. Pat. No. 4,284,656, patented Aug. 18, 1981, disclosed a process for the preparation of a soybean curd product from a defatted soybean material in which a portion of the protein was extracted with water. A low water to defatted soy material ratio was required to obtain an extract with a sufficiently high protein content to precipitate a curd product as desired. Coagulation was accomplished by adjusting the pH to within the range from about 5.4 to about 8.0 and heating the aqueous protein extract to within a temperature range from about 80.degree. C. to about 170.degree. C.
Ultrafiltration is a process or technique for the separation of dissolved materials on the basis of their molecular size and shape by passing the solution through an infinitesimally fine filter. The ultra filter is a tough, thin, selectively permeable membrane which retains most macromolecules above a certain size while allowing the smaller molecules and solvent to pass through as a filtrate. The retained protein macromolecules form the retentate while the smaller sugars, amino acids and salts are removed.
Ultrafiltration then provides a means to remove solvent and salts from a solution of macromolecules without phase changes, temperature extremes, ionic or pH changes which can affect the structure of proteins. In short, it separates the larger molecules from the smaller ones without affecting their structure. High flux membranes have been developed which can retain molecules as small as 500 daltons or as large as 300,000 daltons. Ultrafiltration systems are available in a number of design configurations: spiral wound, hollow fiber, internally-coated tubular and flat leaf systems.
Ultrafiltration used in conjunction with reverse osmosis, another type of membrane process which employs membrane of much smaller pore size, has the advantage of eliminating a waste water effluent, since the reverse osmosis membrane purifies the filtrate to a point where it is cleaner than ordinary tap water. This permits both the recirculation of water previously used to extract the oilseed protein and oil fractions as well as the elimination of a water pollution source. Many prior art processes would be forced to merely discard the unwanted whey (which retains significant amounts of organic matter and other waste constituents) into the environment (Lawhon et al., 1981).
A number of prior art patents describe the use of ultrafiltration membranes in combination with other techniques to separate proteins from unwanted molecules to achieve various end products. Other ultrafiltration techniques have required the addition of various chemical treatments (enzymatic hydrolysis, EDTA etc.) prior to ultrafiltration (Iacobucci et al., U.S. Pat. No. 3,736,147). Others have also required the use of ultrafiltration membranes with prohibitively small molecular weight cut off (MWCO) values which will retain smaller protein molecules with attached phenolic compounds and can result in a bitter-tasting product. One prior art method disclosed for sunflower meal processing required an inert gas blanket (N, He, Ar) prior to membrane ultrafiltration to prevent the formation of off-colors due to extraction of the protein at an alkaline pH (O'Connor U.S. Pat. No. 3,622,556).